Paper products generally in the form of a cellulosic web, such as hand towels, wipers, and the like are manufactured widely in the paper making industry. Each product has unique product characteristics requiring the appropriate blend of product attributes to ensure that a product can be used in its intended use locus. These attributes include tensile strength, water absorbency, caliper, and stretch. Typically, many paper products are made using a throughdrying process. Throughdrying is a common means of drying paper webs using a relatively non-compressive method of removing water from the web by passing hot air through the web until it is dry. More specifically, a wet lade web is transferred from a forming fabric to a coarse, highly permeable throughdrying fabric and retained on the throughdrying fabric until drying is complete. The resulting dried web is softer and bulkier than webs dried by other methods because fewer bonds are formed and because the web is substantially uncompressed. The throughdried product is somewhat stiff and rough to the touch. The throughdried sheet is often directed to other processing steps to convert the material base sheet into value added products by combining base sheets, coating base sheets, or further converting the base sheets into improved materials.
Before discussing the invention, other aspects of the paper industry should be discussed. Extended nip presses have been adapted to paper making processes over the last few years. Typically, extended nip presses are used on board and other non-tissue paper grades. These extended nip presses are used to apply higher loads to sheets for the purpose of calendering dried sheets or dewatering wet lade sheets.
More particularly, extended nip presses are used in the paper industry to press water from a formed web. These systems involve passing a wet paper web between a backing roll and a felt where the felt is supported on one side by an endless belt. A press shoe presses the belt towards the backing roll causing the felt to partially wrap around the arc of the backing roll. The paper web is dewatered as it passes through the extended nip. Extended nip rolls have also been used in soft calendering. See for example, U.S. Pat. No. 5,163,364 to Bubik et al. Surface properties of paper and board passing through a calender nip can be improved. The "supersoft" calender incorporates a relatively soft elastic moving belt supported by a glide or a roll instead of the covered roll in a conventional soft calendar. As a result, smoothness and variation in gloss can be substantially improved using supersoft calender processes in an extended nip. Prior art embossing processes typically involve contacting the cellulosic sheet with common embossing structures involving opposed rolls having matched male and female embossing means.
Typically, a pair of contacting rolls are used to convert a cellulosic web. Traditional embossing methods include a matched pair of steel embossing rolls, arranged to move relative to each other to form a conventional non-extended roll nip. The web is embossed as it passes through the nip. Another method of embossing tissue utilizes oppositely disposed steel and rubber rolls are used to emboss the web. Such embossing is used to increase the bulk and to impart a pattern on the tissue web. In other applications, rolls are used to perforate a web. See for example, U.S. Pat. No. 5,356,506 to McNeil et al., which discloses a patterned roll for converting a paper laminate. The roll has a hollow cylindrical perforate outer shell with a plurality of radially oriented holes through the shell.
At relatively low operational speeds, traditional systems are sufficient to produce a defined and stable embossing pattern. However, with increasing web speeds the embossing pattern is difficult to maintain. Conventional embossing improvements have centered on the configuration of the embossing roll. For example, the embossing pattern has been altered to produce a higher bulk and softer embossed sheet. See U.S. Pat. No. 5,562,805 to Kamps et al., where fine-scale intermeshed embossing elements of two rolls emboss the tissue thereby increasing tissue surface fuzziness which can improve softness.
Other attempts to improve the embossing pattern have involved changing the roll material from traditional steel to a softer material. See U.S. Pat. No. 4,211,743 to Nauta et al., where the embossing rolls have a resilient surface of varying hardness. The resilient surfaces temporarily deform within the nip thereby ensuring that the web material is fully contacted by the embossing pattern. Generally, a heated thermoplastic web is passed between this resilient roll nip. Heating the web makes the web more moldable. However, heating the web is often not preferable or even possible, particularly when the web is cellulosic. Traditional roll-on-roll embossing methods of cellulosic webs continue to have many shortcomings. Throughdried cellulosic webs present additional problems because their resilient tissue structure and high internal bulk inhibit pattern retention. For example, a soft throughdried tissue web as disclosed in U.S. Pat. No. 5,607,551 to Farrington et al. possesses such resilient characteristics. Such webs tends to relax to some degree to its original state when released from the embossing nip.
Another disadvantage associated with traditional roll-on-roll embossing methods is that an operator has limited control over the embossing pattern once a machine is set up. Variables affecting the pattern are relatively fixed. For example, the rolls are in contact for a finite amount of time, so the embossing dwell time is limited. Dwell time is the amount of time that the web material is in contact with the embossing elements. Dwell time is a function of web speed and nip width. Nip width is controlled by roll diameter, nip load, and the roll surface characteristics. Larger rolls, higher loads, and softer roll surfaces will produce a wider nip and result in a longer dwell time. Decreasing web speed and increasing nip width are not realistic alternatives where the trends in the industry are faster web speeds, replaceable components, and shorter machine set-up times. In addition, high nip loads on softer roll surfaces at high web speeds creates heat which adversely affects the embossing process and creates safety concerns.
Therefore, a need exists for an embossing method and apparatus that will produce a retained embossing pattern in a resilient cellulosic web traveling at high web speeds. Moreover, a need exists for the greater control of the embossing process to accommodate different web materials and speeds.